Discussion on the energy-saving application effect of VFD (Variable-frequency Drive) in thermal power enterprises

With the continuous development and improvement of frequency conversion technology, its excellent power-saving performance, good start-up characteristics and various protection functions have been well reflected in enterprises, which have saved energy in production, reduced costs, and increased production costs. ASD.

1. Features of VFD (Variable-frequency Drive)

Frequency conversion speed regulation is the speed regulation technology with the best performance and the most promising development. VFD (Variable-frequency Drive) is the main product used for speed regulation of three-phase asynchronous motors in industrially developed countries. Its main advantages are: good energy saving effect, which can reach more than 55%; wide speed regulation range, speed regulation ratio can reach 20:1; good starting and braking performance, can realize soft start, automatic smooth acceleration and deceleration and fast braking; The protection function is perfect, and it can realize various protection methods such as overvoltage, undervoltage, overload, overcurrent, instantaneous power failure, short circuit, stall, etc., and can realize fault judgment and display; it is easy to use in electronic computer systems and can realize remote control.

2. Application of frequency conversion speed regulation technology in cogeneration system control

2.1 Central heat exchange station and external network control

The boiler system mainly includes system constant pressure water replenishment, system hot water (carrier) circulation, system combustion control and furnace pressure control. In cogeneration, the superheated steam generated by it can be output directly, or hot water can be output after steam-water exchange , the heat energy of steam or hot water is provided to each heat user through each heat exchange station in the pipe network.

2.2 Control of external network circulation pump

Keeping the pressure constant in the heating system is a basic prerequisite for the proper operation of the heating system. The pipe network of the heating system is usually a closed pipe system, in theory, the water consumption is very small. However, the unavoidable "running, dripping, leaking" and man-made water loss in practice will inevitably affect the stability of the system pressure. In addition, the temperature of the water flow in the system also affects the pressure change. In order for the system to supply heat stably, it must be ensured that the highest point of the system is full of water, and the pressure must not exceed the capacity of the system. To do this, it is necessary to determine a constant pressure point on the return water pipe of the system. During the process of system circulation, maintain a stable pressure value, so that the flow and head of the circulating pump are limited within a certain range, and the flow rate can be realized. , Head automatic adjustment, to a certain extent for the boiler system can also avoid overtemperature and overpressure. The water supply pump with variable frequency speed regulation is used to maintain the constant pressure of the system, which can smoothly adjust the speed of the water supply pump, adjust the water supply in time, and ensure the stability of the water volume in the system. The method is to use the pressure sensor to convert the pressure signal from the constant pressure point of the system into a (4~20)mA current signal and send it to the pressure regulator. The regulator will compare it with the preset pressure value and send out a frequency Command to VFD (Variable-frequency Drive), VFD (Variable-frequency Drive) will automatically adjust the speed of the pump motor, and then adjust the amount of water, due to the incompressible characteristics of the liquid, the response to the pressure control of the liquid is fast, it is easy to make constant The pressure at the pressure point is maintained at the set value.

In the heating system, water is the carrier of heat energy, and the transportation of heat energy is completed by the flow of water. The higher the temperature of the water, the higher the heat content per unit of water, and the greater the flow of water, the greater the heat energy transported. For the heat balance of the whole heating network, the frequency conversion speed regulation technology is used to adjust the flow rate of the circulating pump, and the flow rate can be adjusted under the condition of stable system pressure to ensure the safe operation of the heating system. First of all, according to the currently recommended principle of "low outlet water temperature, small temperature difference between outlet and return" and the operation adjustment formula, the outlet water temperature of the system is adjusted according to the change of ambient air temperature. At the same time, the speed of the circulating pump is adjusted according to the temperature difference between the outlet and return water, so as to indirectly adjust the temperature by adjusting the flow of the system. The specific method is to use the temperature sensor to convert the temperature change of the sampling point into a corresponding voltage value or current value and then send it to the regulator. After the regulator compares it with the preset temperature value, it sends a frequency command to the VFD ( Variable-frequency Drive), VFD (Variable-frequency Drive) automatically adjusts the speed of the pump motor.

2.3 Control the high-voltage frequency conversion of the induced draft fan

High-voltage VFD (Variable-frequency Drive) is a series-superimposed high-voltage VFD (Variable-frequency Drive), that is, multiple single-phase three-level inverters are connected in series to output high-voltage alternating current with variable frequency and voltage. According to the basic principles of electromechanics, the speed of the motor satisfies the following relationship:

Among them: P is the number of pole pairs of the motor; f is the operating frequency of the motor; s is the slip). It can be seen from the formula that the synchronous speed n0 of the motor is proportional to the operating frequency of the motor (n0=60f/p). Since the slip s is generally small (0~0.05), the actual speed n of the motor is approximately equal to the synchronous speed of the motor. Speed n0, so the actual speed of the motor can be changed by adjusting the power supply frequency f of the motor. The slip s of the motor is related to the load. The greater the load, the greater the slip, so the actual speed of the motor will decrease slightly with the increase of the load.

VFD (Variable-frequency Drive) itself consists of three parts: transformer cabinet, power cabinet and control cabinet. Three-phase high-voltage electricity enters through the high-voltage switchgear, and supplies power to the power units in the power unit cabinet through input step-down and phase shifting. The power units are divided into three groups, one group is one phase, and the output of the power unit of each phase string. The control unit in the main control cabinet performs rectification, inverter control and detection for each power unit in the power cabinet when passing through the optical fiber, so that the frequency is given through the operation interface according to actual needs, and the control unit sends the control information to the power unit Perform corresponding rectification and inverter adjustments, and output voltage levels that meet the load requirements.

The input side is powered by a phase-shifting transformer for each unit, and the secondary windings of the phase-shifting transformer are divided into three groups; this multi-stage phase-shifting superposition rectification method can greatly improve the current waveform of the grid side, making the grid side under load The power factor is close to 1. In addition, due to the independence of the secondary winding of the transformer, the main circuit of each power unit is relatively independent, similar to a conventional low-voltage VFD (Variable-frequency Drive).

On the output side, the U and V output terminals of each unit are connected in series to form a star connection to supply power to the motor. By reorganizing the PWM waveform of each unit, a ladder PWM waveform can be obtained. This kind of waveform has good sine degree and small dv/dt, which can reduce the insulation damage to cables and motors. It can make the output cable length very long without output filter, and the motor does not need to be derated, so it can be directly used for the transformation of old equipment. ; At the same time, the harmonic loss of the motor is greatly reduced, eliminating the resulting mechanical vibration and reducing the mechanical stress on the bearings and blades.

2.3.1 Operation and start-up of VFD (Variable-frequency Drive)

There are three operation methods of VFD (Variable-frequency Drive): one is local control, directly using the start, acceleration, deceleration, reset, stop and other buttons of the man-machine interface on the touch screen of the control cabinet to realize the operation process of frequency conversion speed regulation